Try it. The worst that can happen is it goes through and you get overdraft/overage fee by your credit card company which will end up costing you way more than $5 for SR4. If it is a prepaid card it will either go through or be declined.
If you have cash, you should be able to go to a CVS or Walgreens and get money put into your Paypal account or add it through Moneypak. I know you can get a Amazon gift card/voucher at those stores, thats if Amazon price matches Steam.
Paypal credit should be like a virtual credit card. Make sure you pay it off when the payment is due or you get charged interest and fees, most likely.